1. Field of the Invention
The present invention relates to a fuel cell, and more particularly, a membrane-electrode assembly for a fuel cell.
2. Discussion of the Related Technology
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen in a hydrocarbon-based material such as methanol, ethanol, or natural gas. Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell, which uses methanol as a fuel.
The polymer electrolyte fuel cell is a clean energy source that is capable of replacing fossil fuels. It has advantages such as high power density and energy conversion efficiency, operability at room temperature, and being down-sized and tightly sealed. Therefore, it can be applicable to a wide array of fields such as non-polluting automobiles, electricity generation systems, and portable power sources for mobile equipment, military equipment, and the like.
The polymer electrolyte fuel cell has an advantage of a high energy density, but it also has problems in the need to carefully handle hydrogen gas and the requirement of accessory facilities such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce hydrogen as the fuel gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the polymer electrolyte fuel cell, but it has the advantages of easy handling of the liquid-type fuel, a low operation temperature, and no need for additional fuel reforming processors. Therefore, it has been acknowledged as an appropriate system for a portable power source for small and common electrical equipment. Such a fuel cell includes a stack composed of unit cells, and it produces various ranges of power output. Since it has a four to ten times higher energy density than a small lithium battery, it is has been highlighted as a small portable power source.
In the above-mentioned fuel cell system, the stack that generates electricity substantially includes several to scores of unit cells stacked adjacent to one another, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly includes an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
The membrane-electrode assembly is generally fabricated as follows: a microporous layer for uniform supply of a reactant is formed on a carbon paper as an electrode substrate; a catalyst layer is coated on a microporous layer to fabricate an electrode; and then the electrode is hot-pressed with an electrolyte membrane. The electrode substrate is uniform and thus is difficult to be assembled with a 25 μm-thick thin electrolyte membrane. Further, the above method has a problem that the catalyst utility efficiency is varied in accordance with a catalyst coating manner. Therefore, in order to use a thin electrolyte membrane and improve catalyst utility efficiency, there has been research into alternative methods such as a method where a catalyst layer coated on an inactive substrate such as polytetrafluoroethylene is hot-pressed with an electrolyte membrane.
The discussion in this section is to provide general background information, and does not constitute an admission of prior art.